Ambient air quality affects the health of people breathing the ambient air. The lower the air quality, the greater the risk for health-related problems induced by the ambient air. Conventional particulate matter monitors measure the mass concentration of particulate matter within ambient air, gases, or other fluids to detect the quality of the ambient air or gaseous fluid. A conventional particulate matter monitoring device can provide a warning to a user when the device detects a relatively low air quality (e.g., a relatively large particulate mass concentration within the air) or a decrease in the ambient air quality based upon an increase in particulate mass concentration measured over a specific time period.
Certain sensing techniques, such as in mechanical resonance sensing, detect the particulate mass concentration of an air sample. For example, a conventional particulate matter monitor includes a collector that collects particulate matter within an air sample and that detects the mass of the particulate matter based upon the mass-spring principle. If the collector is part of a mechanically resonating system, the natural resonance frequency of the system decreases as the collected mass of the particulate matter increases. The general equation governing this behavior is:Δm=m0[(f0/ff)2−1]where Δm is the mass increment of collected particulate matter, m0 is the total initial mass of the resonating system, and f0 and ff are the initial (before particle collection) and final (after particle collection) resonance frequencies of the oscillating system (e.g., of the collector).
One type of particulate matter monitor utilizing mechanical resonance sensing includes a quartz crystal microbalance. Conventional quartz crystal microbalances have a thin piezoelectric quartz crystal sandwiched between two metal electrodes. The conventional quartz crystal microbalances collect particulate matter on the surface of the quartz crystal from an air sample using either electrostatic precipitation or jet-to-plate impaction. As the piezoelectric quartz crystal receives the particulate matter, the quartz crystal microbalance provides an alternating electric field to the piezoelectric quartz crystal, causing the quartz crystal to generate a shear-induced acoustic wave. Changes in the mass of the quartz crystal, as caused by deposition of particulate matter, affect the frequency of the wave. In such a configuration, the quartz crystal microbalance allows for detection of and quantification of relatively small masses of particulate matter within an air sample.
Another type of particulate matter monitor utilizing mechanical resonance sensing includes a tapered-element oscillating microbalance (TEOM). In a conventional TEOM, a filter cartridge attached to a tapered element of the TEOM receives an air or gas sample pumped at a known flow rate. The TEOM produces an alternating voltage signal that oscillates the tapered element at the resonant frequency of the tapered element/filter cartridge combination. As the filter cartridge, attached to the tapered element, removes particulate matter from the air or gas sample, the mass change of the tapered element causes a frequency shift in the resonant frequency of the tapered element. The frequency shift of the signal relates to the mass accumulated by the tapered element/filter cartridge combination of the TEOM and relates to the amount of particulate matter within the air sample.
Another type of particulate matter monitor utilizing mechanical resonance sensing includes a resonant taut filter membrane sensing system. The conventional taut filter membrane sensing system includes an annular piezoelectric driver to impart an oscillation to a taut filter and a miniature microphone to detect the resonance frequency of the filter. The taut filter membrane sensing system detects the mass of particulate matter collected on the taut filter by oscillating the filter, thereby causing the filter to resonate at its natural mechanical resonant oscillation frequency (e.g., characteristic fundamental frequency or fundamental mode). As particulates within an air sample accumulate on the filter, the resonant frequency of the filter's oscillation decreases due to the additional particulate mass. The taut filter membrane sensing system calculates the mass concentration of airborne particulates within the air sample based upon the decrease in the natural resonant frequency of the taut filter.
Additionally, the conventional taut filter membrane sensing system is configured to oscillate the taut filter at higher, non-harmonic modes related to the fundamental mode or characteristic fundamental frequency of the filter. As particulates within an air sample accumulate on the filter, the oscillation of the filter at the higher, non-harmonic modes decreases due to the additional particulate mass. By using the higher, non-harmonic modes to oscillate the taut filter, the resonant taut filter membrane sensing system allows for relatively sensitive measurements of particulate matter concentrations within an air sample, compared to measurements taken using fundamental mode oscillations.